In recent years, in order to catch up with rapidly advancing downsizing of electronic units, it has become increasingly necessary to package semiconductor components for each of these electronic units at a higher and higher density. Correspondingly, sizes and thicknesses of semiconductor components have also been noticeably reduced.
Hereinafter, a conventional resin-molded semiconductor device will be described.
FIG. 23(a) is a plan view of a conventional resin-molded semiconductor device, and FIG. 23(b) is a cross-sectional view of the conventional resin-molded semiconductor device.
As shown in FIGS. 23(a) and 23(b), this conventional resin-molded semiconductor device is of the type including external electrodes on its back surface.
The conventional resin-molded semiconductor device includes a leadframe consisting of: inner leads 201; a die pad 202; and support leads 203 for supporting the die pad 202. A semiconductor chip 204 is bonded onto the die pad 202 with an adhesive, and electrode pads (not shown) of the semiconductor to chip 204 are electrically connected to the inner leads 201 with metal fine wires 205. And the die pad 202, semiconductor chip 204, inner leads 201, support leads 203 and metal fine wires 205 are molded with a resin encapsulant 6. In this structure, no resin encapsulant 206 exists on respective back surfaces of the inner leads 201. In other words, the respective back surfaces of the inner leads 201 are exposed and respective lower parts of the inner leads 201, including these exposed back surfaces, serve as external electrodes 207.
In such a resin-molded semiconductor device, the respective back surfaces of the resin encapsulant 206 and those of the inner leads 201 are both located in the same plane, and the die pad 202 is located above the inner leads 201. That is to say, by providing depressed portions 208 for the support leads 203, the die pad 202 is elevated above the inner leads 201. Thus, after the device has been molded with the resin encapsulant 206, a thin layer of the resin encapsulant 206 is also formed on the back surface of the die pad 202. In FIG. 23(a), the resin encapsulant 206 is illustrated as being transparent such that the inside of the semiconductor device can be looked through. In FIG. 23(a), the semiconductor chip 204 is indicated by the dashed line and the illustration of the metal fine wires 205 is omitted.
Also, conventionally, to secure a required standoff height from the back surface of the resin encapsulant 206 in bonding the external electrodes to the electrodes of a motherboard such as a printed wiring board, on which a resin-molded semiconductor device is mounted, ball electrodes 209 of solder are attached to the external electrodes 207 as shown in FIG. 24. After the standoff height has been secured by these ball electrodes 209, the device is mounted onto the motherboard.
Hereinafter, a method for manufacturing the conventional resin-molded semiconductor device will be described with reference to the drawings. FIGS. 25 through 27 are cross-sectional views illustrating a manufacturing process for the conventional resin-molded semiconductor device.
First, as shown in FIG. 25, a leadframe 210, including the inner leads 201 and die pad 202, is prepared. It is noted that the die pad 202 is actually supported by the support leads, but the illustration of the support leads is omitted in FIG. 25. Depressed portions are formed in the support leads and the die pad 202 is elevated above the plane on which the inner leads 201 are located. The leadframe 210 does not include any tie bar used for preventing the resin encapsulant from flowing out during resin encapsulation.
Next, as shown in FIG. 26, the semiconductor chip 204 is bonded, with an adhesive, to the die pad 202 of the lead-frame prepared. This process step is called "die bonding".
Then, as shown in FIG. 27, the semiconductor chip 204, which has been bonded onto the die pad 202, is electrically connected to the inner leads 201 via the metal fine wires 205. This process step is called "wire bonding". As the metal fine wires 205, aluminum (Al) or gold (Au) fine wires may be appropriately used, for example.
Subsequently, as shown in FIG. 28, the die pad 202, semiconductor chip 204, inner leads 201, support leads and metal fine wires 205 are molded with the resin encapsulant 206. In this case, the leadframe, on which the semiconductor chip 204 has been bonded, is introduced into a molding die assembly and transfer-molded. In particular, resin encapsulation is performed with the back surface of the inner leads 201 in contact with an upper or lower die of the die assembly.
Finally, the ends 211 of the inner leads 201, protruding outward from the resin encapsulant 206, are cut off after the resin encapsulation. By performing this cutting process step, the end faces of the inner leads 201 cut off are substantially flush with the side faces of the resin encapsulant 6 and the respective lower parts of the inner leads 201 are used as external electrodes 207.
In the manufacturing process of this conventional resin-molded semiconductor device, the resin encapsulant 206 might overflow from the edges of the inner leads 201, reach the back surfaces thereof and thereby form resin bur thereon (overflowing resin) during the resin encapsulation process step. Thus, a water jet process step is introduced for blowing off the resin bur after the resin encapsulation process step and before the process step of cutting off the inner leads 201.
Also, if necessary, ball electrodes of solder are formed on the lower surfaces of the external electrodes 207, thereby completing the resin-molded semiconductor device shown in FIG. 24. As another alternative, a solder plating layer is sometimes formed instead of the solder balls.